Heat exchanger

ABSTRACT

A heat exchanger ( 28  includes a first manifold ( 34 ), a second manifold ( 36 ) and a plurality of parallel heat transfer tubes ( 38 ). The first manifold ( 34 ) includes one or more manifold sections ( 58, 60, 63 ). A first one of the manifold sections ( 58 ) is connected to an inlet ( 64 ). The heat transfer tubes ( 38 ) extend between and fluidly connect the first and the second manifolds ( 34, 36 ). At least some of the heat transfer tubes ( 38 ) are disposed with the first manifold section ( 58 ).

Applicant hereby claims priority benefits under 35 U.S.C. §119(e) of U.S. Provisional Patent Application No. 61/318,592 filed Mar. 29, 2010, the disclosure of which is herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates generally to heat exchangers, and particularly to heat exchangers having structures operable to decrease thermal mechanical fatigue therein.

2. Background Information

A typical two-pass microchannel condenser includes a plurality of parallel microchannel tubes that extends between a first manifold and a second manifold. The first manifold includes an inlet section connected to a refrigerant inlet, and an outlet section connected to a refrigerant outlet. The refrigerant inlet is disposed at a top end of the inlet section. During operation, refrigerant is directed into the inlet section of the first manifold through the refrigerant inlet. The inlet section of the first manifold directs the refrigerant through a first set of the microchannel tubes and into the second manifold. The second manifold then redirects the refrigerant through a second set of the microchannel tubes to the outlet section of the first manifold. As ambient air is forced around the microchannel tubes, thermal energy is transferred from the refrigerant in the tubes into the ambient air, thereby cooling the refrigerant.

Depending upon size and geometry of the condenser and the ambient conditions, the temperature of the refrigerant passing through the condenser can be reduced more than 100-150 degrees Fahrenheit (° F.). A temperature drop of this magnitude can subject elements of the condenser (e.g., a section of the first manifold between the inlet section and the outlet section) to a relatively large temperature gradient. Additionally, as the refrigerant is initially distributed from the refrigerant inlet into the first set of the microchannel tubes, the portion of the tubes proximate the top corner of the condenser can also be subjected to a substantial temperature gradient. Substantial temperature gradients within elements of the condenser can result in relatively high thermally induced stresses in those elements. The high stresses can, in turn, cause mechanical fatigue and failure in the elements.

SUMMARY OF THE DISCLOSURE

According to a first aspect of the invention, a heat exchanger is provided that includes a first manifold, a second manifold and a plurality of parallel heat transfer tubes. The first manifold includes an inlet manifold section fluidly connected to an inlet. The inlet manifold section extends along a centerline between a first end and a second end. The heat transfer tubes extend between and fluidly connect the first and the second manifolds. An integer “N” number of the heat transfer tubes are disposed along the centerline between the inlet and the first end of the inlet manifold section. An integer “M” number of the heat transfer tubes are disposed along the centerline between the inlet and the second end of the inlet manifold section. “N” is approximately equal to “M”.

According to a second aspect of the invention, a heat exchanger is provided that includes a first manifold, a second manifold and a plurality of parallel heat transfer tubes. The first manifold includes a plurality of manifold sections. A first one of the manifold sections is disposed adjacent to, and configured discrete from, a second one of the manifold sections. The heat transfer tubes extend between and fluidly connect the first and the second manifolds. At least some of the heat transfer tubes are disposed with the first manifold section. At least some of the heat transfer tubes are disposed with the second manifold section.

According to a third aspect of the invention, a heat exchanger is provided that includes a first manifold, a second manifold and a plurality of parallel heat transfer tubes. The first manifold includes an inlet manifold section connected to an inlet. The heat transfer tubes extend between and fluidly connect the first and the second manifolds. A region of the heat exchanger, between the first and the second manifolds, is adapted having a reduced heat transfer coefficient. This reduced heat transfer coefficient region is disposed proximate the inlet.

The foregoing features and advantages and the operation of the invention will become more apparent in light of the following description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic illustration of an exemplary embodiment of a heat exchange system.

FIG. 2 is a diagrammatic illustration of one embodiment of a heat exchanger for use in the heat exchanger system in FIG. 1.

FIG. 3 is a diagrammatic illustration of another embodiment of a heat exchanger for use in the heat exchanger system in FIG. 1.

FIG. 4 is a diagrammatic illustration of another embodiment of a heat exchanger for use in the heat exchanger system in FIG. 1.

FIG. 5 is a diagrammatic illustration of an enlarged section of the heat exchanger shown in FIG. 4 proximate a refrigerant inlet aperture.

FIG. 6 is a diagrammatic illustration of another embodiment of a heat exchanger for use in the heat exchanger system in FIG. 1.

FIG. 7 is a diagrammatic illustration of a section of the heat exchanger shown in FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, an exemplary heat exchanger system 10 (hereinafter “HX system”) is shown that includes a closed loop refrigerant circuit 12 (hereinafter “refrigerant circuit”). The refrigerant circuit 12 includes a plurality of components including a compressor 14, a condenser 16, an evaporator 18 and, optionally, a subcooler 20. During operation, the HX system 10 shown in FIG. 1 can cool an environment by cycling refrigerant (or coolant) through the refrigerant circuit 12. For example, the compressor 14 directs compressed refrigerant into the condenser 16. As ambient air is forced (e.g., via a fan) through the condenser 16, heat energy is transferred from the refrigerant within the condenser 16 into the ambient air, thereby cooling the refrigerant and heating the ambient air. From the condenser 16, the refrigerant is directed into the subcooler 20. As ambient air is forced (e.g., via a fan) through the subcooler 20, additional heat energy is transferred from the refrigerant within the subcooler 20 into the ambient air. From the subcooler 20, the refrigerant is directed into the evaporator 18. As ambient air is forced (e.g., via a fan) through the evaporator 18, heat energy is transferred from the ambient air into the refrigerant within the evaporator 18, thereby cooling the ambient air and heating the refrigerant. From the evaporator 18, the refrigerant is directed back into the compressor 14. The present invention, however, is not limited to the aforesaid system configuration. In other embodiments, some components may be added or subtracted, and/or arranged in different orders within the refrigerant circuit 12 depending upon design requirements. For example, the HX system 10 can further include an expansion valve 22, a pressure relief valve 24, a liquid-to-liquid heat exchanger 26, etc.

Referring to FIG. 2, an exemplary embodiment is shown of a heat exchanger 28 configured having one or more liquid-to-air (or air-to-liquid) heat exchanger sections 30 and 32. Each heat exchanger section 30, 32 can be adapted to operate as the condenser 16, the subcooler 20 or the evaporator 18 of the HX system 10 in FIG. 1; however, the present invention is not limited to the functionalities of these components. According to an aspect of the present embodiment, a heat exchanger section includes a condenser section 30 adapted to operate as the condenser 16, and a subcooler section 32 adapted to operate as the subcooler 20. The heat exchanger 28 includes a first manifold 34 (sometimes referred to as a “header”), a second manifold 36 and a plurality of heat transfer tubes 38 (hereinafter the “HT tubes”). The heat exchanger 28 can further include a plurality of cooling fins 40, a region 42 having a reduced heat transfer coefficient (hereinafter the “reduced HTC region”) (see FIGS. 4 to 6) and/or a plurality of mounting brackets 44, 46, 48 and 50.

The first manifold 34 extends along a first centerline 52 (e.g., parallel to the y-axis) between two ends 54 and 56. The first manifold 34 includes one or more manifold sections 58, 60, 62 and one or more refrigerant inlet/outlet apertures 64, 66, 68 (hereinafter “I/O apertures”). In the embodiment in FIG. 2, the manifold sections include a condenser inlet section 58, a condenser outlet section 60 and a subcooler inlet section 62. Each manifold section 58, 60, 62 extends along the first centerline 52 between a first (e.g., top) end 54, 70, 72 and a second (e.g., bottom) end 74, 76, 56 defining a height extending therebetween. Each manifold section 58, 60, 62 is fluidly separated from an adjacent manifold section 58, 60, 62 by a baffle 78 or any other suitable fluid flow obstruction. Referring to FIG. 3, in some embodiments, two adjacent manifold sections (e.g., the condenser inlet section 58 and the condenser outlet section 60) can also be physically separated along the first centerline 52 by a distance 80. In this specific embodiment, the condenser inlet section 58 is configured discrete from the condenser outlet section 60 and the subcooler inlet section 62; however, the present invention is not limited to such a configuration.

In the embodiment in FIG. 2, the I/O apertures include a condenser inlet 64, a condenser outlet 66 and a subcooler inlet 68. The condenser inlet 64 is fluidly connected to the condenser inlet section 58. In this specific embodiment, the condenser inlet 64 is disposed approximately halfway between the top and the bottom ends 54, 74 of the condenser inlet section 58; however, the present invention is not limited to this configuration. For example, referring to the embodiments in FIGS. 3-6, the condenser inlet 64 can be disposed proximate the top end 54 of the condenser inlet section 58. Referring again to FIG. 2, the condenser outlet 66 is fluidly connected to the condenser outlet section 60. The subcooler inlet 68 is fluidly connected to the subcooler inlet section 62.

The second manifold 36 extends along a second centerline 82 (e.g., parallel to the y-axis) between two ends 84 and 86. The second manifold 36 includes one or more manifold sections 88, 90 and at least one I/0 aperture 92. In the embodiment in FIG. 2, the manifold sections include a condenser return section 88 and a subcooler outlet section 90. Each manifold section 88, 90 extends along the second centerline 82 between a first (e.g., top) end 84, 94 and a second (e.g., bottom) end 96, 86, defining a height extending therebetween. Each manifold section 88, 90 can be fluidly separated from an adjacent manifold section 88, 90 by a baffle 98 or other suitable flow obstruction. In some applications, the second manifold sections 88 and 90 can be physically separate from one another. In the embodiments shown in FIGS. 2 and 3, the I/O aperture is adapted as a subcooler outlet 92. The subcooler outlet 92 is fluidly connected to the subcooler outlet section 90.

The HT tubes 38 are arranged in parallel rows that extend (e.g., substantially perpendicular to the first and the second centerlines 52 and 82) between the first and the second manifolds 34 and 36. The HT tubes 38 can further be arranged into a plurality of HT tube sets 100, 102, 104. For example, the HT tubes 38 in the heat exchanger 28 in FIG. 2 are arranged into a first set 100, a second set 102 and a third set 104. The first set of HT tubes 100 fluidly connects the condenser inlet section 58 of the first manifold 34 to the condenser return section 88 of the second manifold 36. In this specific embodiment, an integer “N” number of these first set HT tubes 100 (where N>1) are disposed along the first centerline 52 between the condenser inlet 64 and the top end 54 of the condenser inlet section 58. An integer “M” number of these first set HT tubes 100 (where M and N are equal or approximately equal to one another) are disposed along the first centerline 52 between the condenser inlet 64 and the bottom end 74 of the condenser inlet section 58. The present invention, however, is not limited to this specific configuration. The second set of HT tubes 102 fluidly connects the condenser return section 88 of the second manifold 36 to the condenser outlet section 60 of the first manifold 34. The third set of HT tubes 104 fluidly connects the subcooler inlet section 62 of the first manifold 34 to the subcooler outlet section 90 of the second manifold 36.

Referring now to FIG. 7, each HT tube 38 is disposed a distance 106 (e.g., along the first and/or the second centerlines 52, 82) from each adjacent HT tube 38. In some embodiments, one or more of the HT tubes 38 includes a plurality of parallel microchannels 108 disposed within the tubes 38. Examples of suitable microchannel tube configurations are disclosed in U.S. Pat. Nos. 7,281,387 and 7,000,415 both to Daddis, Jr. et al., which are hereby incorporated by reference in their entirety. The present invention, however, is not limited to the aforesaid microchannel tube configuration.

The cooling fins 40 are arranged into a plurality of rows 110, 112 and 114. Each row of cooling fins 110, 112, 114 is respectively disposed between a pair of adjacent HT tubes 38 within each heat exchanger section 30, 32. In some embodiments, a row of cooling fins can also be disposed between a pair of HT tubes 38 that are each disposed in different heat exchanger sections 30, 32 (not shown). The cooling fin 40 in each row 110, 112, 114 that is disposed closest to the first manifold 34 (i.e., the “first fin”) can be adjacent or in contact with the first manifold (e.g., see FIG. 2). Alternatively, the “first fin” 40 in a given row can be disposed a distance 116, 117, 119 (e.g., perpendicular to the first centerline 52) away from, the first manifold 34 (e.g., see FIG. 5). In similar fashion, the cooling fin 40 in a given row (e.g., row 110, 112, 114) that is disposed closest to the second manifold 36 (i.e., the “last fin”) can be adjacent or in contact with the second manifold 36 (e.g., see FIG. 2), or disposed a distance away from the second manifold 36.

Referring now to FIGS. 5 and 7, the cooling fins 40 in each row 110, 112, 114 can be adapted having generally “V” or “A” shaped geometries. Each V-shaped cooling fin extends between a first end 120 and a second end 122, and has a mid-point 124 (i.e., where a slope of the fin reverses) therebetween. Referring to FIG. 4, in some embodiments, adjacent rows of cooling fins 110, 112, 114 can be staggered relative to each other such that, for example, the first ends 120 of the V-shaped cooling fins in a first row 110 are aligned (along the x-axis) with the mid-points 124 of the V-shaped cooling fins in a second row 112. Other suitable examples of cooling fin configurations are disclosed in the afore-referenced U.S. Pat. Nos. 7,281,387 and 7,000,415; however, the present invention is not limited to any particular type or arrangement of cooling fins.

Referring to the embodiments in FIGS. 4 to 6, the reduced HTC region 42 is adapted to reduce thermal gradients and the typically accompanying stress and fatigue. In the specific embodiments in FIGS. 4 to 6, the reduced HTC region 42 is located proximate the condenser inlet 64, between the condenser inlet section 58 of the first manifold 34 and the condenser return section 88 of the second manifold 36.

In the specific embodiment in FIGS. 4 and 5, the reduced HTC region 42 includes a plurality of finless regions 126; i.e., regions between the HT tubes 38 without cooling fins 40. These finless regions 126 are configured to incrementally decrease in size with each tube 38 further away from the condenser inlet 64. For example, the distance 116, 117, 119 disposed between the condenser inlet section 58 and the cooling fin 40 closest thereto in a respective row 110, 112, 114, for Q number of cooling fin rows (where Q >2), is incrementally decreased as the respective cooling fin rows 110, 112, 114 are disposed farther away from the condenser inlet 64 (i.e., as the distance 128, 130, 132 disposed between the respective cooling fin row 110, 112, 114 and the condenser inlet 64 increases). Using the embodiment shown in FIG. 5 as an example, the distance 116 between the condenser inlet section 58 and the “first fin” 40 in the first cooling fin row 110 is greater than the distance 117 between the condenser inlet section 58 and the “first fin” 40 in the second cooling fin row 112, which is greater than the distance 119 between the condenser inlet section 58 and the “first fin” 40 in the third cooling fin row 114, etc. The number of rows used in the reduced HTC can vary depending upon the application. The present invention, however, is not limited to the aforesaid embodiment. The geometry of the HTC region can be varied to suit the application at hand. For example, in other embodiments, sets of two or more of the finless regions can incrementally decrease in size as each set is disposed further away from the condenser inlet, etc.

In the specific embodiment in FIG. 6, the reduced HTC region 42 includes an airflow reduction element 134 that is disposed with the condenser section 30 proximate the condenser inlet 64. The airflow reduction element 134 includes a sheet of material (e.g., insulation) that is semi-permeable or impermeable to airflow. The airflow reduction element 134 may be disposed on one or both sides of the condenser section 30 of the heat exchanger 28.

Referring again to FIG. 2, the mounting brackets 44, 46, 48 and 50 are adapted to secure the heat exchanger 28 as a single unit to a housing (not shown). The number and configuration of the mounting brackets can be varied to suit the application at hand.

Referring to FIGS. 1-6, during operation of the heat exchanger 28, refrigerant provided from the compressor 14 is directed through the condenser inlet 64 and into the condenser inlet section 58 of the first manifold 34. The condenser inlet section 58 distributes the refrigerant into the first set of HT tubes 110.

As set forth above in a prior art heat exchanger, refrigerant distributing into a region of the heat exchanger proximate the refrigerant inlet can be repeatedly subjected to relatively large thermal gradients. As a result, that region can be subjected to stress and fatigue resulting from the thermal gradients. To resolve this problem, some embodiments of the present invention locate the condenser inlet 64 intermediately between the “N” and the “M” numbers of HT tubes 38. By so locating the inlet 64 relative to the HT tubes 38, the refrigerant more uniformly distributes within the HT tubes 110, and thereby decreases the potential for a substantial thermal gradient within the region.

Referring to FIGS. 4 to 6, in other embodiments of the present invention, the potential for a thermal gradient adjacent the condenser inlet 64 is addressed by reducing the heat transfer coefficient in the region 138 proximate the condenser inlet 64; i.e., in the reduced HTC region 42. The heat transfer coefficient of the structure in the HTC region is reduced relative to the remainder of the heat exchanger. As a result, the rate at which thermal energy is transferred between the refrigerant and the ambient air in this region is reduced and the potential for a large temperature gradient is reduced. In the heat exchanger 28 embodiment shown in FIG. 4, for example, the finless regions 126 reduce the outer surface area of the condenser section 30 in the reduced HTC region 42. The reduction in surface area decreases the rate at which heat energy can be transferred between the refrigerant and the ambient air. Additionally, the finless regions 126 and the staggered arrangement of the cooling fins 40 can compensate for thermal growth of the condenser inlet section 58 (e.g., along the first centerline) by permitting relative movement between adjacent HT tubes 38. In the heat exchanger 28 example shown in FIG. 6, a sheet of insulation 134 insulates the tubes in the reduced HTC region 42, and reduces or prevents the flow of ambient air therethrough. The reduction of airflow serves to decrease the heat transfer rate between the refrigerant and the ambient air within the region, thereby reducing the thermal gradient and potential for thermally induced stresses.

Referring again to FIGS. 1-6, the refrigerant flows through the first set of HT tubes 110 and into the condenser return section 88, thereby completing a first pass through the condenser section. The condenser return section 88 redirects the refrigerant, which was collected from the first set of HT tubes 110, into the second set of HT tubes 112. The refrigerant flows through this second set of HT tubes 112 and into the condenser outlet section 60, thereby completing a second pass through the condenser section. The condenser outlet section 60 directs the refrigerant, which was collected from the second set of HT tubes 112, out of the heat exchanger 28 through the condenser outlet 66.

Depending upon size and geometry of the condenser section 30 and ambient conditions, the refrigerant can be cooled more than, for example, 100-150 degrees Fahrenheit (° F.) as it flows through the first and the second passes of the condenser section 30. As set forth above, such a temperature drop can subject a mid section of a prior art manifold, between an inlet manifold section and an outlet manifold section, to a relatively sharp temperature gradient. Referring to FIG. 3, mechanical stresses potentially associated with such a large temperature gradient can be mitigated by providing the physical separation between the condenser inlet section 58 and the condenser outlet section 60. For example, in this configuration, these manifold sections 58 and 60 can independently thermally grow and/or move relative to each other without subjecting the heat exchanger 28 to additional mechanical stresses.

Referring again to FIGS. 1-6, refrigerant is directed back into the heat exchanger 28 through the subcooler inlet 68 and into the subcooler inlet section 62. The subcooler inlet section 62 distributes the refrigerant into the third set of HT tubes 114. The refrigerant flows through this third set of HT tubes 114 and into the subcooler outlet section 90, thereby completing a first pass through the subcooler section. The subcooler outlet section 90 directs the refrigerant, which was collected from the third set of HT tubes 114, back out of the heat exchanger 28 through the subcooler outlet 92.

While various embodiments of the present invention have been disclosed, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible within the scope of the invention. For example, the first and/or the second manifolds 34 and 36 can each include additional manifold sections (e.g., return sections) such that the refrigerant can complete additional passes through one or more of the heat exchanger sections. Accordingly, the present invention is not to be restricted except in light of the attached claims and their equivalents. 

What is claimed is:
 1. A heat exchanger, comprising: a first manifold including an inlet manifold section fluidly connected to an inlet, which inlet manifold section extends along a centerline between a first end and a second end; a second manifold; and a plurality of parallel heat transfer tubes extending between and fluidly connecting the first and the second manifolds, wherein “N” number of the heat transfer tubes are disposed along the centerline between the inlet and the first end of the inlet manifold section, wherein “M” number of the heat transfer tubes are disposed along the centerline between the inlet and the second end of the inlet manifold section, and wherein “N” and “M” are integers and “N” is approximately equal to “M”.
 2. The heat exchanger of claim 1, wherein the first manifold further includes a second manifold section that extends along a centerline, and wherein at least some of the heat transfer tubes are disposed along the centerline of the second manifold section.
 3. The heat exchanger of claim 2, wherein the inlet manifold section is disposed adjacent to the second manifold section, and wherein the inlet manifold section is discrete from the second manifold section.
 4. The heat exchanger of claim 3, wherein the inlet manifold section is spaced a distance away from the second manifold section.
 5. The heat exchanger of claim 2, wherein the inlet manifold section and the second manifold section are fluidly separated by a baffle.
 6. The heat exchanger of claim 1, wherein a region of the heat exchanger, between the first and the second manifolds, is adapted having a reduced heat transfer coefficient, which reduced heat transfer coefficient region is disposed proximate the inlet.
 7. The heat exchanger of claim 6, further comprising a plurality of rows of cooling fins respectively disposed between adjacent rows of the heat transfer tubes, wherein a distance is disposed between the first manifold and a respective cooling fin closest thereto for Q number of rows, wherein the distance is incrementally decreased as the respective rows are disposed farther away from the inlet, and wherein Q is an integer greater than
 2. 8. The heat exchanger of claim 7, wherein the cooling fins in adjacent rows are staggered relative to one another.
 9. The heat exchanger of claim 6, wherein the reduced heat transfer coefficient region includes an airflow reduction element that is disposed with the heat transfer tubes proximate the inlet.
 10. A heat exchanger, comprising: a first manifold including a plurality of manifold sections, a first one of the manifold sections being disposed adjacent to, and configured discrete from, a second one of the manifold sections; a second manifold; and a plurality of parallel heat transfer tubes extending between and fluidly connecting the first and the second manifolds, wherein at least some of the heat transfer tubes are disposed with the first manifold section, and wherein at least some of the heat transfer tubes are disposed with the second manifold section.
 11. The heat exchanger of claim 10, wherein the first manifold section is spaced a distance away from the second manifold section.
 12. The heat exchanger of claim 10, wherein the first manifold section and the second manifold section are fluidly separated by a baffle.
 13. The heat exchanger of claim 10, wherein the first manifold section is connected to an inlet.
 14. The heat exchanger of claim 13, wherein the first manifold section extends along a centerline between a first end and a second end, wherein “N” number of the heat transfer tubes are disposed along the centerline between the inlet and the first end of the first manifold section, and wherein “M” number of the heat transfer tubes are disposed along the centerline between the inlet and the second end of the first manifold section, wherein “N” and “M” are integers and “N” is approximately equal to “M”.
 15. The heat exchanger of claim 10, wherein a region of the heat exchanger between the first and the second manifolds has a reduced heat transfer coefficient, and which region is disposed proximate the inlet.
 16. The heat exchanger of claim 15, further comprising a plurality of rows of cooling fins respectively disposed between adjacent rows of the heat transfer tubes, wherein a distance is disposed between the first manifold and a respective cooling fin closest thereto for “Q” number of rows, wherein this distance is incrementally decreased as the respective rows are disposed farther away from the inlet, and wherein “Q” is an integer greater than
 2. 17. The heat exchanger of claim 15, wherein the reduced heat transfer coefficient region includes an airflow reduction element that is disposed with the heat transfer tubes proximate the inlet.
 18. A heat exchanger, comprising: a first manifold including an inlet manifold section connected to an inlet; a second manifold; and a plurality of parallel heat transfer tubes extending between and fluidly connecting the first and the second manifolds; wherein a region of the heat exchanger, between the first and the second manifolds, is adapted having a reduced heat transfer coefficient, which reduced heat transfer coefficient region is disposed proximate the inlet.
 19. The heat exchanger of claim 18, further comprising a plurality of rows of cooling fins respectively disposed between adjacent rows of the heat transfer tubes, wherein a distance is disposed between the first manifold and a respective cooling fin closest thereto for “Q” number of rows, wherein this distance is incrementally decreased as the respective rows are disposed farther away from the inlet, and wherein “Q” is an integer greater than
 2. 20. The heat exchanger of claim 18, wherein the reduced heat transfer coefficient region includes an airflow reduction element that is disposed with the heat transfer tubes proximate the inlet.
 21. The heat exchanger of claim 18, further comprising a plurality of rows of cooling fins respectively disposed between adjacent rows of the heat transfer tubes, and wherein the reduced heat transfer coefficient region includes a plurality of finless regions disposed between the adjacent rows of the heat transfer tubes proximate the inlet. 